1. Field of the Invention
This invention relates to a bringing-up apparatus disposed in a fry channel for accommodating the fry of salmon, such as pink salmon, masou salmon, trout, char or the like, particularly yolk sac fry in an early stage of development after breeding, and more particularly, to a bringing-up apparatus for providing a repose place for the yolk sac fry in the fry channel.
2. Description of the Prior Art
The incubation of salmon and similar fish usually comprises a breeding step, in which fertilized eggs are bred, a repose period protection step for protecting the yolk sac fry in a fry channel where fresh water flows gradually at all times, and a feed step, in which free swimming fry are fed in a free swimming period after the repose period.
The yolk sac fry during the repose period repose in a repose place in the fry channel where there is a gentle water flow, without substantially taking bait and also without moving. Therefore, in a fry channel where only fresh water flows gradually at all times, the yolk sac fry concentrate excessively in particular places such as the corners of the fry channel because the place where the water flow is gentle, i.e., place of repose of the yolk sac fry is scant in the fry channel. When a particular repose place is excessively crowded by the yolk sac fry, oxygen that is necessary for the yolk sac fry becomes insufficient which results in defective growth or death of the fry. For this reason, the capacity of the fry channel for the yolk sac fry has to be reduced.
Therefore, it is a usual practice to employ a fry channel, where small gravel free from the corners are laid over the bottom of the channel and fresh water flows gently at all time with a minimum depth of 7 to 8 cm and at a velocity of 3 cm/sec. The aim of this fry channel is to provide places where the water flow is gentle by laying gravel over the bottom to increase the place of repose of the yolk sac fry at the bottom so as to distribute the yolk sac fry over the bottom, provide sufficient oxygen to the distributed yolk sac fry and thus increase the capacity of accommodating the yolk sac fry.
Actually, however, the conditions of the place of repose provided by the gravel varies with the shape, size, amount, distribution or the like of the gravel that is laid. Therefore, there is a trend that the yolk sac fry concentrates in the best place of repose even in the fry channel as noted above, so that defective growth or death of the yolk sac fry due to insufficiency of available oxygen occurs. For this reason, it is necessary to reduce the capacity of accommodation of the yolk sac fry.
Further, the fry channel is cleaned after the free swimming fry have entered the free swimming stage after the repose period and have been transferred to a feeding channel for the next step. Where the gravel is laid over the fry channel, however, the gravel must be washed one by one. For these reasons, the cleaning requires a long time in addition to the fact that the cleaning and handling of gravel is very cumbersome. Further, sometimes the gravel which is to be laid over the fry channel bottom is not readily available.
Further, if the free swimming fry having attained the free swimming period after the repose period is not transferred to the feeding channel for the next step but the fry channel is used directly as the feeding channel, the bait for the free swimming fry is trapped among or attached to the gravel laid over the fry channel bottom because of the gravels remaining in the channel. The bait which thus remains in the channel corrodes or produces fur to contaminate the channel. In this case, the free swimming fry are more liable to catch diseases.